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Monthly Service Tips

What Do the numbers Mean?


The tire size branded on the sidewall provides a significant amount of information about the tire's intended purpose, dimensions, load capacity and high temperature/high speed durability.

Our primary example will be based on variations of the 225/50R16 size, although other sizes will appear where appropriate.

Service Type

Most tire sizes begin with a letter or letters that identify the type of vehicle and/or type of service for which they were designed. The common indicators are as follows:

P225/50R16 91S

P = When a tire size begins with a "P," it signifies the tire is a "P-metric" size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. This includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity). The use of P-metric sizes began in the late 1970s and they are the most frequently used type of tire size today.

225/50R16 92S

If there isn't a letter preceding the three-digit numeric portion of a tire size, it signifies the tire is a "Metric" size (also called "Euro-metric" because these sizes originated in Europe). While Metric tire sizes are primarily used on European cars, they are also used on vans and sport utility vehicles. Euro-metric sizes are dimensionally equivalent to P-metric sizes, but typically differ subtly in load carrying capabilities.

T125/90D16 98M

T = If a tire size begins with a "T," it signifies the tire is a "Temporary Spare" ("space saver" or "mini spare") that was designed to be used temporarily only until a flat tire can be repaired or replaced.

LT245/75R16 108/104S

LT = If a tire size begins with "LT," it signifies the tire is a "Light Truck-metric" size that was designed to be used on vehicles that are capable of carrying heavy cargo or towing large trailers. This includes medium and heavy-duty (typically 3/4- and 1-ton load capacity) pickup trucks, sport utility vehicles and full-size vans. Tires branded with the "LT" designation are the "little brothers" of 18-wheel tractor-trailer tires and are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.

7.50R16LT 112/107Q, 8.75R16.5LT 104/100Q or 31x10.50R15LT 109Q

LT = If a tire ends with "LT," it signifies the tire is either an earlier "Numeric", "Wide Base" or "Flotation" Light Truck size designed to be used on vehicles that are capable of carrying heavy cargo and towing trailers (Numeric sizes), use 16.5-inch diameter rims (Wide Base sizes) or are wider, oversized tires designed to help the vehicle drive on top of loose dirt or sandy surfaces (Flotation sizes). This includes light, medium and heavy duty (typically 1/2-, 3/4 and 1-ton load capacity) pickup trucks and sport utility vehicles. Tires branded with the "LT" at the end of their size designation are also the "little brothers" of 18-wheel tractor-trailer tires and are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.

195/70R15C 104/102R

C = If a Euro-metric sized tire ends with a "C," it signifies the tire is a "Commercial" tire intended to be used on vans or delivery trucks that are capable of carrying heavy loads. In addition to being branded with the "C" in their size, these tires are also branded with their appropriate Service Description and "Load Range" (Load Range B, Load Range C or Load Range D).

ST225/75R15

ST = If a tire size begins with "ST," it signifies the tire is a "Special Trailer Service" size that was designed to only be used on boat, car or utility trailers. ST-sized tires should never be used on cars, vans or light trucks.


How Do I Compare Price vs. Value?

Why is it that the price of fuel seems relatively inexpensive while the cost of its tires may give the impression of being high? If we tracked our total costs we'll find that typical fuel costs for just 10, 000 miles of driving for a car that delivers 20 mpg (see chart below) actually exceed our tire costs. And believe us, we've found that the quality and performance of tires has a lot more to do with driving satisfaction than fuel does.

We think that much of the misperception has to do with the fact that we buy fuel one tank-full at a time, and don't really look at its total cost for thousands of miles. Our tires are paid for "up front" and then last for tens of thousands of miles.

If you are selecting new tires to improve your car's performance and come across two tires that have caught your eye, comparing their cost over the long run can help you decide. If one tire looks perfect, although more expensive than another tire that appears to be a close second, consider evaluating your options by comparing "how much per mile" each tire will cost.

Consider the following:

If you drive your sports car another 20,000 miles and are considering the "perfect" set of performance tires at $150 each, or another set that costs $130 each, you may be surprised to find out that the cost of the "perfect" set of four tires costs just 3 cents per mile...while the "second place" set of four costs 2.6 cents per mile. Will not spending the extra $80 today make up for not having selected the "perfect" tire that you will be driving on the next couple of years?

If you drive your vehicle another 60,000 miles and are considering "premium" long-wearing tires at $100 each, or economy tires costing $60 each, you may be surprised to find out that the cost of the "premium" set costs just 0.6 cents per mile...while the economy set costs 0.5 cents per mile. Will not spending the extra $100 today make up for not having selected the "premium" tire that you will be driving on the next four years? Will the economy tire last 60,000 miles?

If you live in the snowbelt and drive your vehicle another 60,000 miles and are considering 2 sets of tires (a set for summer and winter) at $100 each, or all-season tires costing $75 each, you may be surprised to find out that the cost of both "premium" sets costs just 1.3 cents per mile total...while the economy set costs 0.5 cents per mile. Will not spending the extra $500 today make up for not having selected the premium summer and winter tires that will make it easier to navigate snow and ice for the upcoming years?

Estimated Total Fuel Costs:
10,000 miles @ 15 mpg = 666 gal @ $3.37/gal = $2,244.00
10,000 miles @ 20 mpg = 500 gal @ $3.37/gal = $1,685.00
10,000 miles @ 25 mpg = 400 gal @ $3.37/gal = $1,348.00
10,000 miles @ 30 mpg = 333 gal @ $3.37/gal = $1,122.00


Air Pressure - Correct, Underinflated and Overinflated

Advantages of Correct Tire Inflation

Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles. Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire's structure, blending the tire's responsiveness, traction and handling.

Can you easily identify which tire is 30% underinflated? Here is what they would look like in the morning parked in your garage.

 


Tough to tell; isn't it? Tire pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.

Disadvantages of Underinflation

An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle's tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn't seem excessively low, remember, it usually represents about 20% of the tire's recommended pressure.

Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.


When Should I Replace My Tires?

While it is simple to place a Lincoln penny in the tread groove of a worn tire and use Abe's likeness as a guide, we don't think Honest Abe knows what's best for today's drivers.

The Lincoln penny tread depth test has been touted for years to be a suitable method of determining when it's time to get new tires. It's based on the premise you're driving on legal tread depths anytime the top of Lincoln's head is obscured by the tread and that a tire's ability to grip the road isn't greatly reduced in adverse conditions (rain, slush and snow) until the tread wears to about 2/32" of remaining depth. It then maintains you're ready for new tires at 2/32" and can see the penny above Lincoln's head. (See Photo #1) Is it as simple as that? No. Read on.

Honest Abe Doesn't Always Tell The Truth

Photo #1: Honest Abe doesn't always tell the truth.

A driver's ability to control their vehicle depends on the traction between their tires and the road. Tires don't require tread designs or even much tread depth to deliver traction on dry roads. A practical example of this is the racing slicks used on stock cars and open-wheel racers that provide traction at over 200 mph. However, tires do require tread designs to generate traction on wet, slushy and snow-covered roads. Liquids can't be compressed and require time and energy to move them out of the way as our tires drive through them. Those same racing slicks would lose traction at amazingly slow speeds anytime something prevented them from maintaining contact with the road.

So a tread design is necessary to direct water and slush from between the tire and the road, as well as provide edges that bite into snow. But that's only half the equation; because we've seen that tread depth also contributes to how well the design does its job.

The air our tires encounter at highway speeds can easily be compressed and moved out of the way with relative ease. However the same isn't true of liquids. When water collects on the road surface during rainstorms, the water depth, vehicle speed and vehicle weight, as well as the tires' tread designs and tread depths collectively determine when and if the tires will be forced to hydroplane and how quickly they can stop a vehicle.

A typical passenger car tire has about twenty square inches of total footprint surface and begins with about 1/3" of tread depth. While the majority of the footprint surface is made up of the rubber that grips the road, the remainder is the void of the grooves that make up the tread design.

Obviously the tread will wear away over the life of the tire and the volume of its tread grooves will be reduced. While this occurs so slowly that it may not be noticed day-to-day, ultimately there will be a time when the driver will notice the car slip in the snow, hydroplane in the rain or simply not stop in as short a distance on wet roads.

In order to confirm how much wet traction worn tires sacrifice, members of the service team measured the stopping distances from 70 mph (the typical speed limit of U.S. Interstate highways) with vehicles equipped with sets of new tires and compared them to tires with about 4/32" (3mm) of remaining tread depth, followed by sets with the legal minimum of 2/32" (1.6mm) depth. The differences surprised us! Vehicles equipped with the 2/32" minimum tire tread depth took about 100 more feet to stop and were still traveling at about 45 mph at the same distance the vehicles equipped with the 4/32" deep tires had already come to a complete stop!

Our advice is that if rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, you need enough tread depth to allow the rain to escape through the tire's grooves. If the water can't escape fast enough, your vehicle's tires will be forced to hydroplane (float) on top of the water, losing traction and increasing stopping distances.

Additionally, if snow-covered roads are a concern, you should consider replacing your tires when they reach approximately 6/32" of remaining tread depth to maintain good mobility. You need more tread depth in snow because your tires need to compress the snow in their grooves and release it as they roll. If there isn't sufficient tread depth, the "bites" of snow your tires can take on each revolution will be reduced to "nibbles," and your vehicle's traction and mobility will be sacrificed.

While replacing your tires before they are legally worn out may not appear the most economical practice, it is far less expensive than repairing your car if it can't stop in an emergency situation in less distance than the vehicle ahead of you!